This post from Psyblog is a list of psychological observations to do with memory. Some observations reveal unintuitive patterns in memory, like this one:
If you want to learn to play tennis, is it better to spend one week learning to serve, the next week the forehand, the week after the backhand, and so on? Or should you mix it all up with serves, forehands and backhands every day?
It turns out that for long-term retention, memories are more easily recalled if learning is mixed up. This is just as true for both motor learning, like tennis, as it is for declarative memory, like what’s the capital of Venezuela (to save you googling: it’s Caracas).
The trouble is that learning like this is worse to start off with. If you practice your serve then quickly switch to the forehand, you ‘forget’ how to serve. So you feel things are going worse than if you just practice your serve over-and-over again. But, in the long-run this kind of mix-and-match learning works best.
One explanation for why this works is called the ‘reloading hypothesis’. Each time we switch tasks we have to ‘reload’ the memory. This process of reloading strengthens the learning.
That does make sense, actually. I recall noticing on one occasion that it was quite effective to learn a language whilst doing another task (i.e. returning to the language exercise regularly).